Numerous forms of air cleaners have been produced for separating larger particles of dust and the like from incoming air. For example, the U.S. Pat. No. 1,934,311 granted Nov. 17, 1933 to Cegerreis et al refers to air entering the outer housing of the air cleaner which is given a swirling motion by a series of fixed angular vanes through which the air is drawn from the outer atmosphere. The air leaving the air cleaner passes through a central opening which is concentric with the outer housing. As the air passes through the inlet, it is increased in speed by a funnel-shaped deflector which gradually increases the air velocity in a direction parallel to the axis of the inlet. At the point where the air reverses direction to flow through the outlet, the air comes into contact with a series of angularly spaced defuser blades which tend to stop the swirling movement of the air. As a result, the larger particles of dust and other impurities continue their movement in an axial direction and flow into a dust pocket where these larger particles are trapped.
U.S. Pat. No. 1,734,030 issued Nov. 5, 1929 to Bennett shows a somewhat similar arrangement in which the air is directed in a swirling movement by passing through fixed louvers or vanes in the inlet of the air cleaner. The outlet passage, which is concentric with the inlet, is formed by the flaring shape of the outlet so that the velocity of the air reaches its peak as the air passes the large diameter extremity of the outlet. The extremity of the outlet is provided with upwardly and outwardly inclined vanes which tend to break up the swirling movement of the air at the point where the velocity is at its peak. Accordingly, where the air changes its direction to flow through the outlet, the entrained heavier particles of dust and other impurities continue their generally axial movement and are collected in a dust collecting chamber.
U.S. Pat. No. 3,670,480 issued July 20, 1972 to Petersen also shows an air cleaner having a tubular outlet or discharge passage encircled by a concentric inlet. The incoming air is directed in a spiral passage by angularly spaced vanes which act to rotate a spinner mounted co-axially within the chamber. The spinner is designed to increase the normal tendency of the swirling air to flow outwardly toward the peripheral outer wall of the housing. As a result, solid particles of dust and the like which are entrained in the moving air are directed through an outlet passage in the outer wall of the housing. U.S. Pat. No. 3,973,937 issued Aug. 10, 1976 to Petersen makes use of a paddle-type impeller instead of a spinner. In neither of the Petersen patents is provision made for adjusting the velocity of air flow with changes in engine speed.